In the production of terephthalic acid by a liquid phase oxidation of paradialkylbenzene, terephthalic acid produced will deposit within the mother liquor as crystals forming a slurry containing crystals of terephthalic acid. By separating the crystals out of this slurry, a crude product of terephthalic acid is obtained. After dissolution of the resulting crude crystals and subjecting to purification by treatments by oxidation, reduction and so on, terephthalic acid is caused to deposit as crystals to form a slurry containing the crystals. By separating the crystals from this slurry, purified terephthalic acid is obtained.
In either of the above-mentioned cases, the slurry has heretofore been subjected to procedures of separating the crystals by filtration or by centrifugation, washing the so separated crystals by re-slurrying in a wash liquor, separating again the crystals from this re-slurry and, if necessary, repeating these operations. Such a conventional technique reveals disadvantages that the procedures are complicated and that a larger apparatus has to be installed therefor.
As an alternative for this technique, a method had been proposed in which a sucking filtration, filter cake washing and a further sucking filtration are successively effected within a rotary sucking filter (designated hereinafter as RSF. In this technique, a sequence of operations of a sucking filtration, a filter cake washing, a further sucking filtration and removal of the filtered cake is realized in succession while rotating a cylindrical filter medium (filter cloth), wherein an ordinary pressure prevails outside the filter medium and the filtration pressure is attained by sucking out of the internal of the filter medium. However, this technique has a disadvantage that the stuffing or blockage of the filter medium proceeds quite steeply and the operation of apparatus is forced to be interrupted.
The object of the present invention is to obviate the disadvantage mentioned above and to propose a novel method for recovering crystals from a slurry under the use of RSF, which enables to recover the crystals by effecting a sequence of operations of filtration, washing and further filtration without suffering from stuffing of the filter medium.